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Blue Diversion Autarky – Wastewater Treatment off the Grid

Autarky at a glance

The Blue Diversion Autarky toilet is a sanitation system which provides hygiene and comfort without relying on water and wastewater infrastructure. Water, urine and feces are collected separately and treated on site in specific modules. The Blue Diversion Autarky toilet recycles water for hand washing and flushing, recovers nutrients for fertilizer production and inactivates pathogens reliably. The treatment modules can be replaced, integrated in other sanitation systems or used as stand-alone units. One example is the hand washing station, which was also developed by the Blue Diversion Autarky team.

Blue Diversion Autarky was launched in the context of the “Reinvent the Toilet Challenge” funded by the Bill & Melinda Gates Foundation. The project is currently in its second project phase. Blue Diversion Autarky is the continuation of the Blue Diversion project.

Blue Diversion Autarky toilet

Blue Diversion Autarky handwashing station

Guiding principles

Safety & comfort

The Blue Diversion Autarky toilet offers the safety and comfort of a modern flush toilet without requiring piped water or sewerage.

Source separation

Pathogen inactivation, nutrient recovery and water recycling for flushing and hand washing are achieved by the separate treatment of feces, urine and water.

Modularization

A modular design allows for a wide range of applications. Single modules can be used alone or combined with other technologies. One example is the stand alone hand washing station.

Research at a glance

The Blue Diversion Autarky system is based on the separate treatment of water, urine and feces. The core process for the water treatment is a gravity-driven membrane (GDM) filtration. After post-treatment in the form of an activated carbon filter and an electrolysis unit to remove microorganisms, the same water can be reused for flushing and hand washing. Urine is pretreated to eliminate malodor and pathogens, as well as to avoid the loss of nutrients. The volume of urine is then reduced by evaporating the water. The product is a concentrate of organic and inorganic nutrients, that can be used as fertilizer. The treatment of the feces is based on hydrothermal oxidation (HTO). Using high temperature and pressure, the feces are mineralized to carbon dioxide, water and precipitated inorganic solids.

More about water treatment

The water treatment system, referred to as Water Wall, recycles hand washing water, toilet flush water (separated from the major part of urine and feces), or both. A multi-barrier approach with four treatment stages ensures that the water is safe for reuse.

The first barrier and core of the treatment is an aerated bioreactor, in which microorganisms degrade contaminants such as soap, urine, and feces.

As the second barrier, the water is then filtered by gravity through an ultrafiltration membrane. The membrane pores are smaller than the size of bacteria and most viruses, providing water that is microbiologically safe after filtering through the membrane.

Contacts

The combined biological treatment and membrane filtration without cleaning will produce a biofilm on the membrane. This effect is desirable in a gravity-driven membrane (GDM) approach, as the biofilm also contributes to the removal of organic carbon. The GDM system achieves approximately 95% removal of organic carbon.

The filtered water is stored in a clean water tank. Additional treatment is required to limit regrowth of pathogens and potential contamination.

The third treatment barrier, an activated carbon filter, removes remaining organic contamination in the clean water tank by adsorption.

Finally, the fourth and last barrier, an electrolysis unit, further reduces organic carbon concentrations and produces a chlorine residual, both of which help to limit pathogen growth during storage.

Current activities

With this setup, we were able to develop a small series of Water Wall prototypes for treating and recycling hand washing water or toilet flush water. These prototypes were extensively tested under laboratory conditions, under which they reliably removed pathogens, nutrients, malodor, and color from recycled water. However, when moving outside the laboratory, we expect a much higher variability of the number of users, composition of the water and external conditions. This is why we are currently exposing different configurations of the Water Wall to real-life conditions.

A Water Wall as part of a complete toilet system. The biological treatment takes place in the lower tank, with the ultrafiltration membrane sitting on the bottom of the tank. The activated carbon filter and the electrolysis unit are located in the upper tank.

Publications

On-site biological hand washing water treatment can improve global access to safe hand washing water, but requires a thorough understanding of the chemical composition of the water to be treated, and an effective treatment strategy. This study first presents a detailed characterization of the individual inputs to hand washing water. We demonstrate (1) that soap is likely the most significant input in hand washing water, representing ∼90% of mass loading, and (2) that inputs to hand washing water have low concentrations of biologically-essential macro- and micro-nutrients (nitrogen, phosphorus, potassium, copper, zinc, molybdenum and cobalt) with respect to carbon, which may impair biological carbon removal. This study next formulates a recipe that recreates a representative composition of hand washing water and develops a procedure to identify and supplement nutrients in which this recipe is estimated to be deficient. Batch testing of the nutrient-supplemented hand washing water with an inoculum of planktonic bacteria demonstrated improved assimilable organic carbon removal (99% vs. 86% removal) and produced lower final DOC concentrations (1.7 mgC/L vs. 3.5 mgC/L) compared to realistic (nutrient-deficient) washing water. Supplementing nutrients did promote cell growth (50x higher final total cell count). Full-scale testing in a biologically activated membrane bioreactor (BAMBi) system treating 75 L/day of nutrient-supplemented hand washing water showed that long-term operation (100 days) can deliver effective carbon removal (95%) without detrimental fouling or other disruptions caused by cell growth. This work demonstrates that biological treatment in a BAMBi system, operated with appropriate nutrient-balancing offers an effective solution for decentralized treatment of light greywater.

Controlling bacterial pathogens in water for reuse: treatment technologies for water recirculation in the Blue Diversion Autarky Toilet

The Blue Diversion AUTARKY Toilet is a urine-diverting toilet with on-site treatment. The toilet is being developed to provide a safe and affordable sanitation technology for people who lack access to sewer-based sanitation. Water used for personal hygiene, hand washing, and flushing to rinse urine- and feces-collection bowls is treated, stored, and recycled for reuse to reduce reliance on external water supplies. The system provides an opportunity to investigate hygiene of water for reuse following treatment. Treatment in the toilet includes a Biologically Activated Membrane Bioreactor (BAMBi) followed by a secondary treatment technology. To identify effective secondary treatment, three options, including granular activated carbon (GAC) only, GAC+chlorine (sodium hypochlorite), and GAC+electrolysis are considered based on the bacterial inactivation and growth inhibition efficiency. Four different hygiene-relevant bacteria are tested: Escherichia coli, Enterococcus faecalis, Pseudomonas aeruginosa, and Salmonella typhimurium. Our evaluation demonstrates that—despite treatment of water with the BAMBi—E. coli, P. aeruginosa, and S. typhimurium have the potential to grow during storage in the absence of microbial competition. Including the indigenous microbial community influences bacterial growth in different ways: E. coli growth decreases but P. aeruginosa growth increases relative to no competition. The addition of the secondary treatment options considerably improves water quality. A column of GAC after the BAMBi reduces E. coli growth potential by 2 log10, likely due to the reduction of carbon sources. Additional treatments including chlorination and electrolysis provide further safety margins, with more than 5 log10 inactivation of E. coli. However, reactivation and/or regrowth of E. coli and P. aeruginosa occurs under in the absence of residual disinfectant. Treatment including the BAMBi, GAC, and electrolysis appear to be promising technologies to control bacterial growth during storage in water intended for reuse.

On-site treatment of used wash-water using biologically activated membrane bioreactors operated at different solids retention times

Biologically activated membrane bioreactors (BAMBis) were operated at suspended solids retention times (SRT) of 7 and 102 days and at full solids retention. The effect of these different approaches of operation on substrate and nutrient conversion, and on permeate flux was investigated. Variations in organic loads and aeration intensities were also studied. Permeate flux stabilized during long-term operation independently of suspended SRT. Removal of organic substrate was independent of solids concentrations and remained stable over the long term. Microorganisms colonizing the surface of particles were found as the main mechanism responsible for degradation of organic substrate in the particulate form. BAMBi appeared to be a robust technology, adapted to on-site treatment of used wash-water, as it can be operated without control of suspended SRT. Thus BAMBis can be operated for long periods without any control of biofouling and sludge formation, leading to low maintenance needs. When BAMBis were operated at low aeration, the formation of anoxic zones led to combined nitrification and denitrification and thus significant nitrogen removal.

An energy-efficient membrane bioreactor for on-site treatment and recovery of wastewater

The present study describes the development of a new type of aerated membrane bioreactor referred to as a biologically activated membrane bioreactor (BAMBi) for on-site treatment of high-strength wastewater. The treated wastewater is reused for flushing and personal hygiene. BAMBi is an adaptation of a gravity-driven membrane reactor, originally developed for the purpose of treating river water to drinking water quality. Initially, a series of reactor configurations were tested and it was found that the simplest possible configuration could treat the wastewater to an acceptable standard, provided that a polishing step for color removal and disinfection was introduced. A commercial electrolysis unit was utilized for polishing. The energy consumption of BAMBi is 0.8 kWh/m3 of water treated, which can be considered low for an on-site membrane bio reactor application.

More about urine treatment

Two processes are necessary to treat the source-separated urine in the toilet: urine stabilization and water removal.

The main goal of the Autarky urine stabilization is the prevention of urea hydrolysis, a process converting urea to volatile ammonia and carbon dioxide. Through the addition of calcium hydroxide to fresh urine, the pH increases to values above 12 and thus prevents microbial urea hydrolysis.

Contacts

Additionally, the high pH kills pathogens and prevents biological processes that produce malodor. When calcium hydroxide is added to the urine only about the amount dissolves that is needed to reach the necessary high pH value. This allows providing a depot of the reagent in the stabilization reactor; thus, no expensive and complicated dosage mechanisms are required. Moreover, calcium hydroxide, also known as hydrated lime, is a cheap reagent and readily available worldwide.

The direct application of human urine as fertilizer is a common practice in many rural areas around the globe. However, the high water content of urine – no matter if stabilized or not – requires significant storage capacity and can make the collection and transport to the agricultural fields very costly. Volume reduction does not only reduce costs for storage and transport, but could also facilitate field application of the concentrated fertilizer. Standard volume reduction techniques are most often energy intensive processes, because they require high temperature (distillation) or pressure (reverse osmosis).

Offering an alternative to these processes, our approach uses forced convection to reduce the volume of the urine. The evaporation reactor consists of a stacked tray system to create a large surface area. Fans generating a high air stream accelerate the evaporation of the the water from the incoming urine. The offgas is filtered through an activated carbon filter, preventing the emission of organic contamination or malodour.

Once a month, the system needs to be serviced. Tasks are to refill the calcium hydroxide depot and to harvest the trays. The remaining end-product is a concentrate of inorganic and organic nutrients that can be used as fertilizer in agriculture.

Current activities

The stabilization and evaporation reactors have been subject to extensive testing. These tests were supported by lab tests of specific parts of the reactors and a computer model simulating the system. Based on the collected information, the two reactors underwent a major redesign, allowing for an increased evaporation efficiency along with reduced overall size. Another focus of the redesign was the integration of service appliances, e.g. for harvesting of the produced nutrient concentrate.

To examine the long-term functioning, field tests of the urine module as part of the Autarky toilet system were started at Eawag campus Dübendorf in late summer 2018. Further field tests of the Autarky toilet in a household in a Durban township (South Africa) are in preparation. Additionally, the urine module will also be tested as a stand-alone treatment unit in a movable tiny house.

The urine module built-in the Blue Diversion Autarky toilet

Publications

In this study, we investigated the prevention of enzymatic urea hydrolysis in fresh urine by increasing the pH with calcium hydroxide (Ca(OH)2) powder. The amount of Ca(OH)2 dissolving in fresh urine depends significantly on the composition of the urine. The different urine compositions used in our simulations showed that between 4.3 and 5.8 g Ca(OH)2 dissolved in 1 liter of urine at 25 °C. At this temperature, the pH at saturation is 12.5 and is far above the pH of 11, which we identified as the upper limit for enzymatic urea hydrolysis. However, temperature has a strong effect on the saturation pH, with higher values being achieved at lower temperatures. Based on our results, we recommend a dosage of 10 g Ca(OH)2·L−1 of fresh urine to ensure solid Ca(OH)2 always remains in the urine reactor which ensures sufficiently high pH values. Besides providing sufficient Ca(OH)2, the temperature has to be kept in a certain range to prevent chemical urea hydrolysis. At temperatures below 14 °C, the saturation pH is higher than 13, which favors chemical urea hydrolysis. We chose a precautionary upper temperature of 40 °C because the rate of chemical urea hydrolysis increases at higher temperatures but this should be confirmed with kinetic studies. By considering the boundaries for pH and temperature developed in this study, urine can be stabilized effectively with Ca(OH)2 thereby simplifying later treatment processes or making direct use easier.

Urea hydrolysis and precipitation dynamics in a urine-collecting system

Blockages caused by inorganic precipitates are a major problem of urine-collecting systems. The trigger of precipitation is the hydrolysis of urea by bacterial urease. While the maximum amount of precipitates, i.e. the precipitation potential, can be estimated with equilibrium calculations, little is known about the dynamics of ureolysis and precipitation. To gain insight in these processes, we performed batch experiments with precipitated solids and stored urine from a urine-collecting system and later simulated the results with a computer model. We found that urease-active bacteria mainly grow in the pipes and are flushed into the collection tank. Both, bacteria and free urease, hydrolyse urea. Only few days are necessary for complete urea depletion in the collection tank. Two experiments with precipitated solids from the pipes showed that precipitation sets in soon after ureolysis has started. At the end of the experiments, 11% and 24% of urea was hydrolysed while the mass concentration of newly formed precipitates already corresponded to 87% and 97% of the precipitation potential, respectively. We could simulate ureolysis and precipitation with a computer model based on the surface dislocation approach. The simulations showed that struvite and octacalcium phosphate (OCP) are the precipitating minerals. While struvite precipitates already at low supersaturation, OCP precipitation starts not until a high level of supersaturation is reached. Since measurements and computer simulations show that hydroxyapatite (HAP) is the final calcium phosphate mineral in urine solutions, OCP is only a precursor phase which slowly transforms into HAP.

More about feces treatment

In the Blue Diversion Autarky toilet, the feces are separated from the flush water and collected in a container at the bottom of the toilet. They may contain pathogens and must be inactivated quickly to avoid anaerobic decomposition and the emission of malodorous gases.

Contact

In our approach, the organic matter of the fecal sludge is completely mineralized to carbon dioxide, water and minerals such as phosphate salts. The remaining streams are off-gas and a mixture of water and minerals. The off-gas contains mainly nitrogen, carbon dioxide and oxygen. This mixture can be safely vented to the atmosphere. The aqueous stream may be utilized as a fertilizer.

The process used to treat the feces is called “hydrothermal oxidation” or HTO. When mixed with air and heated above around 400°C under high pressure, the fecal sludge decomposes and is oxidized completely to carbon dioxide and water. The water in the sludge does not evaporate but mixes with the air and provides a reaction environment for an efficient conversion of the organic matter within a few minutes.

Current activities

For a better understanding of the HTO process, we are developing a comprehensive computer model of the reactor. This computer model is fed with data from experiments carried out in small autoclaves with real fecal sludge. We determined that the oxidation reaction is rapid above around 300°C and runs to completion within only a few minutes at 400°C.

We are currently improving the third prototype generation of the so-called FOX reactor (short for feces oxidation). This reactor will be field tested with real users soon. We are also working on the design of a more compact version of the FOX reactor, so it can be tested as part of the complete Blue Diversion Autarky toilet in the future.

Team

The Blue Diversion Autarky team consists of researchers and experts from the Swiss Federal Institute of Aquatic Science (Eawag), the Paul Scherrer Institue (PSI), the University of Applied Sciences and Arts Northwestern Switzerland (FHNW) and from EOOS. The project is supported by an advisory board of six international commercial partners. Funding is provided by the Bill and Melinda Gates Foundation (BMGF).